The present invention is directed to a hearing aid having an input transducer, a useful signal processing means having an output stage, and an output transducer, whereby the time-discrete useful signal processing that is controlled by a sampling signal precedes the output stage.
U.S. Pat. No. 4,187,413 discloses a hearing aid of the type initially cited. An analog-to-digital converter and a digital-to-analog converter are arranged preceding an output stage and following an input transducer (microphone). Further, a sampling frequency is provided for the time-discrete signal processing. The sampling frequency is selected to be no lower than twice the upper limit frequency of the useful signals to be processed.
U.S. Pat. Nos. 4,689,819 and 4,592,087 disclose a hearing aid wherein time-discrete useful signal processing with a pulse duration modulation is provided in the output stage, this pulse duration modulation being formed by means of a delta signal. The delta signal is acquired via an integrator that is driven by a square-wave signal from a square-wave generator. The frequency of the square-wave signal and, thus, the frequency of the delta signal as well lie in the ultrasound frequency range. As long as the oscillator frequency is at least twice the frequency of the highest audio frequency component, the resulting frequency spectrum has an audio frequency part that represents an image of the voltage that is generated by a microphone, as well as ultrasound frequency components that are derived from the switch effect that is induced by an oscillator. The output stage is fashioned as a switching amplifier.
The book, Halbleiterschaltungstechnik, 6th Edition, Tietze and Schenk, Springer-Verlag, Berlin, 1983, page 462, discloses an oscillator that is fashioned as a delta voltage generator, whereby the delta signal thereof can be compared to upper and lower threshold voltages in a Schmitt trigger allocated to the oscillator. When the delta signal coincides with one of the threshold voltages, the Schmitt trigger outputs a signal to an analog switch. The oscillator has an integrator whose integration direction is reversible dependent on an input signal supplied via the analog switch. The integrator has its output side in an interactive connection with the Schmitt trigger, whereby the Schmitt trigger controls the analog switch with which the input signal for reversing the integration direction at its integrator output can be supplied to the integrator.
It has been perceived that, first, a combination of various, time-discrete useful signal processing methods in only one hearing aid can be advantageous and that, second, the required outlay for generating the various auxiliary signals (for example, sampling signal and delta signal) is relatively high and leads to space problems in the relatively small hearing aid.
It has also been found that the two auxiliary signals that are required at a minimum can produce disturbing influences in the two different, time-discrete useful signal processing methods. In particular, additional frequencies arise due to the auxiliary signals on the basis of intermodulation and these additional frequencies can fall into the frequency range of the useful signals to be processed either directly or by mixing. A mutual shielding and/or decoupling, for example by filters, of the relevant components for the various, time-discrete useful signal processing methods would thus noticeably increase the circuit-oriented outlay as well as the space requirement in the hearing aid once again. Insofar as such hearing aides are realized, the disadvantages thus arising must be accepted.